It is well known that good modulation extinction ratio waveforms are necessary for power-efficient implementations of optical transmitters (TXs) and receivers (RXs) for many reasons. Extinction ratio (ER), defined as the power off-to-on ratio, determines the initial distance between logical ‘0’ and ‘1’ power levels in on-off keying (OOK) modulation and may lead to multiple interferometric cross talk, which adversely impacts both time-division-multiplexed (TDM) and WDM systems (L. Fenghai, C. J. Rasmussen, and R. J. S. Pedersen, “Experimental verification of a new model describing the influence of incomplete signal extinction ratio on the sensitivity degradation due to multiple interferometric crosstalk,” IEEE Photon. Technol. Lett., vol. 11, p. 137, January 1999; Z. Li, Y. He, B. F. Jorgensen, and R. J. Pedersen, “Extinction ratio effect for high-speed optical fiber transmissions,” in Int. Conf on Comm. Tech. Proc. (ICCT '98), 1998). Moreover, incomplete ER significantly degrades the sensitivity in optically preamplified receivers (S. B. Alexander, Optical communication receiver design. Bellingham, Wash., USA: SPIE Optical Engineering Press, 1997; M. Pauer and P. J. Winzer, “Impact of Extinction Ratio on Return-to-Zero Coding Gain in Optical Noise Limited Receivers,” IEEE Photon. Technol. Lett., vol. 15, pp. 879-881, June 2003; H. Kim and A. H. Gnauck, “Chirp characteristics of dual-drive mach-zehnder modulator with a finite dc extinction ratio,” IEEE Photonics Tech. Lett., vol. 14, March 2002; D. O. Caplan, B. S. Robinson, R. J. Murphy, and M. L. Stevens, “Demonstration of 2.5-Gslot/s optically-preamplified M-PPM with 4 photons/bit receiver sensitivity,” in OFC: Paper PDP23, 2005; D. O. Caplan, “A technique for measuring and optimizing modulator extinction ratio,” in CLEO, 2000).
For example, a 10 dB ER causes ˜3 dB signal-to-noise ratio (SNR) penalty, which, in turn, may reduce the maximum link distance between repeaters or regenerators in fiber-optic applications by a factor of two. An ER better than 20 dB is needed to limit sensitivity penalties to <˜0.7. When using low-duty-cycle-M-ary orthogonal modulation formats (e.g., M-PPM), the ER must be >˜15 dB higher than M to avoid performance penalties. For example, a 128-PPM TX with a 20 dB ER incurs a transmitter power penalty exceeding 3 dB. This penalty is reduced to ˜0.1 dB for a 36 dB ER.
There are several conventional approaches used to generate intensity modulated optical signals, but all suffer from limitations in speed, ER, and/or power efficiency. To achieve high ER, external modulators, such as electro-absorption (EA) and Mach-Zehnder modulators (MZMs), are typically employed (sometimes in cascade (D. O. Caplan, B. S. Robinson, R. J. Murphy, and M. L. Stevens, “Demonstration of 2.5-Gslot/s optically-preamplified M-PPM with 4 photons/bit receiver sensitivity,” in OFC: Paper PDP23, 2005; N. W. Spellmeyer, D. O. Caplan, and M. L. Stevens, “Design of a 5-Watt PPM transmitter for the Mars Laser Communications Demonstration,” in LEOS, 2005) or in parallel (T. Kawanishi, T. Sakamoto, M. Tsuchiya, and M. Izutsu, “70 dB extinction-ratio LiNbO3 optical intensity modulator for two-tone lightwave generation,” in OFC, 2006)), at the expense of increased complexity, component count, and power consumption. While MZMs are often wide band, they are generally polarization dependent, require bias control, and are lossy, increasing transmission (TX) insertion loss by typically 3 to 6 dB. They also have switching voltages of 3 to 6 V, which requires power-hungry external driver amplifiers that typically consume 2 to 5 W of electrical power. EAs are smaller and typically require 3 V drive (60 mA through 50Ω), but only achieve ER>˜10 dB. Higher ER may be achieved with EAs, but at the cost of additional drive voltage and power.
Conventional direct-drive laser current modulation sources, also referred to as a directly modulated lasers (DMLs), are widely used for their cost and simplicity, but typically achieve ER<10 dB and require large drive current, typically ˜100 mA. This requires 5 V drive to accommodate a standard 50Ω load impedance, which precludes the use of standard low-power logic gates for applications at Gbit/s rates. More recently, direct-drive laser techniques in conjunction with optical filtering have been employed to reduce drive power requirements and improve waveform fidelity and ER (R. S. Vodhanel, A. F. Elrefaie, R. E. Wagner, M. Z. Iqbal, J. L. Gimlett, and S. Tsuji, “Ten-to-Twenty Gigabit-per-Second Modulation Performance of 1.5-pm Distributed Feedback Lasers for Frequency-Shift Keying Systems,” J. Lightwave Tech., vol. 7, pp. 1454-1460, October 1989; D. Mahgerefteh, P. S. Cho, J. Goldhar, and H. I. Mandelberg, “Penalty-free propagation over 600 km of nondispersionshifted fiber at 2.5 Gb/s using a directly laser modulated transmitter,” in CLEO, 1999; S. Chandrasekbar, C. R. Doerr, L. L. Buhl, Y. Matsui, D. Mahgerefteh, X. Zheng, K. McCallion, Z. Fan, and P. Tayebati, “Repeaterless Transmission With Negative Penalty Over 285 km at 10 Gb/s Using a Chirp Managed Laser,” Photonics Tech. Lett., vol. 17, pp. 2454-2457, November 2005; S. Chandrasekbar, A. H. Gnauck, G. Raybon, L. L. Buhl, D. Mahgerefteh, X. Zheng, Y. Matsui, K. McCallion, Z. Fan, and P. Tayebati, “Chirp-Managed Laser and MLSE-RX Enables Transmission Over 1200 km at 1550 nm in a DWDM Environment in NZDSF at 10 Gb/s Without Any Optical Dispersion Compensation” Photonics Tech. Lett., vol. 18, pp. 1560-1562, July 2006; Y. Matsui, D. Mahgerefteh, X. Zheng, C. Liao, Z. F. Fan, K. McCallion, and P. Tayebati, “Chirp-Managed Directly Modulated Laser (CML),” Photonics Tech. Lett., vol. 18, pp. 385-386, January 2006; R. S. Vodhanel, A. F. Elrefaie, M. Z. Iqbal, R. E. Wagner, J. L. Gimlett, and S. Tsuji, “Performance of directly modulated DFB lasers in 10-Gb/s ASK, FSK, and DPSK lightwave systems,” J. Lightwave Tech., vol. 8, pp. 1379-1386, 1990). However, the best reported ER with these approaches is <14 dB (D. Mahgerefteh, P. S. Cho, J. Goldhar, and H. I. Mandelberg, “Penalty-free propagation over 600 km of nondispersion shifted fiber at 2.5 Gb/s using a directly laser modulated transmitter,” in CLEO, 1999; S. Chandrasekbar, C. R. Doerr, L. L. Buhl, Y. Matsui, D. Mahgerefteh, X. Zheng, K. McCallion, Z. Fan, and P. Tayebati, “Repeaterless Transmission With Negative Penalty Over 285 km at 10 Gb/s Using a Chirp Managed Laser,” Photonics Tech. Lett., vol. 17, pp. 2454-2457, November 2005; S. Chandrasekbar, A. H. Gnauck, G. Raybon, L. L. Buhl, D. Mahgerefteh, X. Zheng, Y. Matsui, K. McCallion, Z. Fan, and P. Tayebati, “Chirp-Managed Laser and MLSE-RX Enables Transmission Over 1200 km at 1550 nm in a DWDM Environment in NZDSF at 10 Gb/s Without Any Optical Dispersion Compensation” Photonics Tech. Lett., vol. 18, pp. 1560-1562, July 2006; P. A. Morton, G. E. Shtengel, L. D. Tzeng, R. D. Yadvish, T. Tanbun-Ek, and R. A. Logan, “38.5 km error free transmission at 10 Gbit/s in standard fibre using a low chirp, spectrally filtered, directly modulated 1.55 μm DFB laser,” Electron. Lett., vol. 33, pp. 310-311, 1997; M. C. Nowell, J. E. Carroll, R. G. S. Plumb, D. D. Marcenac, M. J. Roberston, H. Wickes, and L. M. Zhang, “Low-Chirp and Enhanced-Resonant Frequency by Direct Push-pull Modulation of DFB Lasers,” IEEE Sel. Top. J. Quantum Electron., vol. 1, pp. 433-441, June 1995). Filtered DMLs with lower drive voltage of 0.5 to 1.5 V are commercially available, but guarantee only 9 dB ER (Finisar, “10 Gb/s 200 km Telcom CML 13 pin-GPO Butterfly Transmitter,” Product Specification 2007).